Method of operating a liquid oxidizer feed piston



March 1968 B. CROSSWHlTE ETAL 3,374,623

METHOD OF OPERATING A LIQUID OXIDIZER FEED PISTON Filed NOV. 16, 1965 m Il\ Billy L. Crosswhife' John F Kinney,

INVENTORS. X144? )1), BY M .1. W M M JM N. M

United States Patent 3,374,623 METHOD OF OPERATING A LIQUID OXIDIZER FEED PISTON Billy L. Crosswhite, Decatur, Ala., and John F. Kinney,

Atlanta, Ga., assignors, by direct and mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Nov. 16, 1965, Ser. No. 508,175 4 Claims. (Cl. 60--39.48)

ABSTRACT OF THE DISCLOSURE A fuel-rich gas-driven piston for feeding liquid oxidizer in a rocket propulsion system is improved by providing in the pressure chamber behind the piston a volatile or thermally decomposable material which releases an inert gas at a temperature of about 200 to 1000 F. The inert gas, which is released upon contact with the hot, fuel-rich press-urizing gas mixture, blankets the chamber and decreases the intensity of reaction of the fuel-rich gas with small amounts of oxidizer in the chamber. The temperature increase due to this reaction is minimized, and the possibility of an explosive reaction is avoided.

The invention described herein may be used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to rocket propulsion systems and more particularly to an improved method of operating gas-actuated pistons for feeding liquid oxidizers.

Inhibited concentrated nitric acid solutions are employed as storable liquid oxidizers in rocket propulsion systems. Oxidizers of this type can be stored in a metal tank on board the rocket for an extended period prior to firing. One widely used storable liquid oxidizer is inhibited red fuming nitric acid, commonly referred to as IRFNA, which normally comprises by weight approximately 84 percent nitric acid, 14 percent nitrogen dioxide, 2 percent water and a small amount of hydrofluoric acid.

In firing the rocket motor, the liquid oxidizer and a liquid fuel are fed under pressure through suitable valves and piping into the combustion chamber. In some applications, the pressure for feeding the oxidizer and fuel is provided by gas-actuated pistons in the oxidizer and fuel tanks, the gas for operation of the pistons being supplied by combustion of a solid-propellant charge in a gas generator. The composition of the gas-generator propellant is selected to provide the desired burning characteristics, in particular, a relatively low flame temperature of about 2000 to 3000 F. This temperature is normally achieved by use of a fuel-rich solid propellant composition, that is, insufficient oxidizer is provided in the mixture for complete oxidation of the resulting gases. The gaseous mixture produced by such a gas-generator is therefore fuelrich, a typical mixture comprising by weight 69 percent carbon monoxide, 1 percent methane, 3 percent hydrogen, 9 percent carbon dioxide, 6 percent water and 12 percent nitrogen.

A serious problem has arisen in the operation of oxidizer feed pistons actuated by gas-generator produced gases. Although a seal is provided between the edge of the piston and the wall of the oxidizer tank, small amounts of the liquid oxidizer remain on the edge of the wall or leak past the head of advancing piston. When the hot, fuel-rich gaseous mixture comes incontact with the oxidizer in this region, a vigorous, exothermic reaction occurs. As a result, the temperature behind the piston is rapidly accelerated to an excessive level so that the temperature limitations of the tank wall are exceeded. The reaction can also occur explosively so as to rupture the tank wall and cause failure of the missile.

ice- Designing the oxidizer tank wall to withstand the increased temperature would result in a substantial weight penalty. Achievement of a completely leak-free seal does not appear feasible, and since only minute amounts of oxidizer are required for'the reaction, complete assurance of reliable operation would be lacking even if the seal were improved.

It is, therefore, an object of this invention to provide a method of operating a liquid oxidizer feed piston actuated by a hot, fuel-rich gaseous mixture wherein the reaction of leaked-in oxidizer with the fuel-rich gases propelling the piston is controlled.

Another object is to provide a means of preventing an excessive temperature increase in the region behind said piston owing to the reaction of small amounts of oxidizer with hot fuel-rich piston-propelling gases.

Still another object is to provide a method of minimizing the reaction of nitric acid and oxides of nitrogen with a hot gaseous mixture containing carbon monoxide, hydrogen and methane.

Other objects and advantages will be apparent from the following detailed description.

In the present invention, the operation of a fuel-rich-gas driven piston for feeding liquid oxidizer in a rocket propulsion system is improved by providing in the pressure chamber behind the piston a volatile or thermally decomposable material which releases an inert gas at a temperature of about 200 to 1000 F. The inert gas, which is released upon contact with the hot, fuel-rich pressurizing gas mixture, blankets the chamber and decreases the intensity of reaction of the fuel-rich gas with small amounts of oxidizer in the chamber. The temperature increase due to this reaction is minimized, and the possibility of an explosive reaction is avoided.

The invention is illustrated by the accompanying drawing wherein FIGURE 1 is perspective view partly in section of a missile utilizing a gas-actuated liquid oxidizer feed piston, and FIGURE 2 is an enlarged sectional view of the oxidizer feed piston mechanism for the missile shown in FIGURE 1.

Referring now to FIGURE 1, there is shown a missile 1 having a pointed nose cone 2 at the forward end and a combustion chamber 3 at the aft end. The cylindrical middle portion of the missile is provided with an annular fuel tank 4 in the upper region thereof. A solid-propellant gas generator 5 is disposed axially within the fuel tank, the gas generator and fuel tank being separated by a tubular wall 6. Bulkhead 7 serves as the bottom of fuel tank 4 and provides structural strength. An annular oxidizer tank 8 is disposed below bulkhead 7, the oxidizer tank being penetrated axially by tubular fuel feed line 9 through which the liquid fuel is conveyed to the combustion chamber. A piston 10, mating with the oxidizer tank wall 11 and fuel feed line 9, is provided at the top of the oxidizer tank. The piston is shown partially advanced, the region behind the piston forming a gas pressurizing chamber 12. The piston is actuated by a fuel-rich gas mixture produced by combustion of a solid propellant charge in the gas generator 5, the gas mixture being introduced into the chamber through inlet 13.

In FIGURE 2, heat-sensitive containers 14 containing finely divided inert-gas-releasing material 15 are shown disposed in the pressurizing chamber 12 between the piston and the bulkhead. In operation of the missile, the solid propellant charge in gas generator 5 is ignited, and combustion of the charge produces a hot, fuel-rich gaseous mixture. The gaseous mixture comes in contact with diaphragm 19 across inlet 13, and the diaphragm is ruptured by the resulting gas pressure, allowing the mixture to enter chamber 12 through inlet 13. Containers 14 are ruptured by contact with the hot pressurized gas, and the inert-gas-releasing material 15 is dispersed in chamber 12. The increased pressure in chamber 12 forces piston 10, downward. The edge of t-hepistonis provided with a static seal 16 and a dynamic seal 18 to preventleakage of liquid oxidizer into chamber 12. A downward-projecting cutter 17 is disposed on the lower edge of piston above dynamic seal 18. As the piston moves downward, the static seal is broken by the cutter. Despite the presence of dynamic seal 18, which wipes oxidizer tank wall as the piston moves downward, small amounts of oxidizer remain on the wall or leak past the seal and enter chamber 12. Thermal decomposition of the inert-gas-releasing material floods the chamber with inert gas and minimizes the reaction of the leaked-in oxidizer with the fuel-rich gas. Liquid fuel is fed from tank 4 through, inlet 20 and line 9 to the combustion chamber by means of a second piston (not shown) located at the forward end of the fuel tank. The operation of the oxidizer feed piston is not affected by the fuel-feeding system operation. The liquid oxidizer is fed into the combustion chamber through an outlet at the aft end of the oxidizer tank and suitable valves and piping (not shown).

Although this invention is not to be understood as limited to a particular theory, it is postulated that the rapid increase in temperature produced when small amounts of IRFNA come in contact with a hot fuel-rich gas mixture behind the piston results primarily from the reaction of nitrogen oxides with carbon monoxide and hydrogen and with methane produced from these gases by a water-gas reaction mechanism. At some tempera-' ture, probably about 1000 F., the reaction system is shifted into an accelerating condition wherein all the fuel or oxidizer in the system will 'be consumed almost instantaneously. By placing an inert-gas-releasing material in the chamber, the system becomes flooded with inert gas before this condition can be reached. Thermal decom: position or volatilization of this material expends some of the heat of reaction and prevents attainment of the temperature required for rapid acceleration of the reaction.

The material placed behind the piston can be any substance which releases inert gas upon being heated to a temperature of about 200 to 1000 F. As used herein, inert gas is intended to refer to any gas which does not react exothermically to a significant extent with the reactive gases in the pressurizing chamber, primarily nitric acid, oxides of nitrogen, carbon monoxide, hydrogen and methane. Carbon dioxide, nitrogen and Water vapor are suitable gases for this purpose. Water alone can be used, the water being vaporized by contact with the fuel-rich gases, but storage in the chamber would be inconvenient and for some applications, the temperature and pressure of the fuel-rich gas mixture would be unduly lowered because of the high heat of vaporization. Numerous types of compounds which undergo thermal decomposition at the specified temperature and release carbon dioxide, nitrogen or water maybe employed. Preferred groups of compounds are alkali metal bicarbonates, alkali metal oxalates and alkali metal alums. The alkali metal bicarbonates rapidly release carbon dioxide at a relatively low temperature and have the further advantages of low cost and ease of handling. Potassium bicarbonate is particularly preferred. Alkali metal oxalates, exemplified by potassium oxalate monohydrate, and their hydrates also release carbon dioxide at a relatively low temperature. Alums, exemplified by potassium aluminum sulfate, KAl(SO '12H O, release water of hydration at a relatively low temperature. Nitrogen-evolving compounds, such as alkali metal azides and nitrides, comprises another group of'suitable compounds. The above-mentioned compounds can also be used in combination with a minor proportion of water, for example, an aqueous slurry containing 2 parts potassium bicarbonate by weight per one part water.

The inert-gas-releasing material is provided in the cham-' her behind the piston in an amount sufiicient to produce an inert gas concentration of at least about-60 mole percent of the mixture obtained upon decomposition or volatilization in the presence of the fuel-rich gas mixture. When diluted to this extent, the reactive fuel and oxidizer gases cannot undergo a rapidly accelerating reaction. The required amount for a given propulsion system depends upon the composition of the fuel-rich gas mixture, the volume of the chamber behind the piston and the composition of the liquid oxidizer. In a particular embodiment for a missile having an oxidizer tank 22 inches in diam eter and an expanded volume of 22,000 cubicinches in the chamber behind the oxidizer feed piston, the pistonactuating gas containing 70 to 75 weight percent reactive gases (carbon monoxide, hydrogen and methane) and the oxidizer being I RFNA, about 9 to 10 pounds of potassium bicarbonateis preferred.

The inert-gas-releasing material is placed so that it will come into contact with the fuel-rich gas mixture in the chamber behind the piston. In a preferred embodiment, the material, in finely divided powder form, is disposed in heat sensitive containers such as thin plastic bags near the pressurizing gas inlet. The hot incoming gas will then rupture the bags and disperse the powder throughout the chamber for effective contact. Containment of the inertgas-releasing material is not critical, however, and it can be deposited loosely in the chamber in any convenient physical form, for example, as an aqueous slurry or finely divided powder.

The present invention is broadly applicable to liquid oxidizer feed systems wherein an oxidizer feed piston is actuated by fuel-rich gases produced by combustion of a gas-generator composition. For any combination of oxidizer and fuel-rich gas mixture, the inert gas would be helpful in minimizing the temperature increase due to reaction thereof. Although this invention is not to be understood as so limited, the liquid oxidizers for which this reaction presents difiiculty are primarily concentrated nitric acid solutions and liquified oxides of nitrogen. Specific oxidizers falling within this group are red fuming nitric acid, that is, nitric acid containing 5 to 20 weight percent nitrogen dioxide dissolved therein; White fuming nitric acid, which is concentrated nitric acid containing 2 percent water and impurities; and liquid nitrogen tetroxide (N 0 The composition of .the fuel-rich gas mixture propelling the piston can vary widely; however, the fuel-oxidizer reaction presents difliculty only when reactive fuel gases comprise over about 30 mole percent of the gaseous mixtures. The fuel-rich mixture for which this invention is applicable are primarily those containing a major proportion such as 60 to 70 weight percent carbon monoxide, along with lesser proportions of hydrogen and methane. Such mixtures are produced by combustion of gas-generator compositions containing relatively small amounts of oxidizer, a typical composition commonly referred to as AGJ comprising 57.6 weight percent nitrocellulose, 20.8 weight percent nitroglycerin, 3.5 weight percent ballistic modifiers, 2.4 weight percent ethyl centralite, 6.2 weight percent dioctyl phthalate and 9.5 weight percent dimethyl sebacate.

The use of inert-gas-releasing material serves primarily to prevent a significant temperature increase in the pressurizing chamber due to the oxidizer-fuel gas reaction. The missile can thus be designed to withstand the temperature and pressure produced by the desired gas-generator composition without allowing for the fuel-oxidizer reaction in the chamber. In a particular embodiment as depicted in the accompanying drawings, the fuel-rich gas enters the chamber at a temperature of about 1900 F.

and builds up pressure to a level of 1200 pounds per square inch, excess pressure being controlled by means of a pressure relief valve. The hot gas, in the absence of a fuel-oxidizer reaction, cools to about 500 F. during operation of the piston for a period of up to seconds. The temperature of the oxidizer tank wall is increased to about 275 F. Under these conditions, a fuel gas-oxidizer reaction in the chamber can increase the gas temperature to a peak of 5000 F. and the Wall temperature to 400 P. which temperature increase, for metals such as aluminum, sharply reduces the strength of the wall. Such temperature increases, as well as the possibility of an explosion, are eliminated by the present invention.

This invention is further illustrated by the following examples.

Example I The effect of inert-gas-releasing material on the peration of a fuel-rich-gas driven oxidizer feed piston was tested by the following procedure. A full-scale model of the oxidizer feed system for the missile depicted in the accompanying drawing was employed, the oxidizer tank having an inside diameter of 21.4 inches and a length of 60 inches. The tank wall was constructed three inches thick to provide strength for oxidizer-fuel reaction testing. The oxidizer tank was filled with IRFNA liquid oxidizer, and the oxidizer feed piston was actuated by ignition of an AGJ gas-generator charge, the composition of these materials being given above. Temperature and pressure in the chamber behind the piston were measured throughout the course of each test for a duration of over 50 seconds. In a test without the added inert-gasreleasing material, the temperature in the chamber rose to over 2500 F. Within less than one second and dropped to 1200 F. within eight seconds. Under the same conditions, except that a slurry comprising two pounds sodium bicarbonate and one pound of water was inserted between the piston and the tank bulkhead, the temperature rose to a peak of only about 1200 F. Maximum pressure was maintained at about 1200 psi. by a pressure relief valve.

Example 11 Tests were conducted using the same oxidizer tank and feed apparatus as is used in actual flight. In these tests, the gas-generator was fired, and temperature and pressure in the chamber behind the piston were measured. The missile was rotated at a speed of four revolutions per second to simulate flight conditions. Without added inertgas-releasing material, the temperature in the chamber rose to 1900 F. in less than one second and dropped to 1200 F. in six seconds. With the same potassium bicarbonate-water charged as in Example I, the temperature rose to a peak of only about 1100 F. in six seconds and dropped to 950 F. in ten seconds.

Example III In test-stand firings of actual missile rounds, the peak temperature in the chamber behind the oxidizer feed piston was decreased about 500 F. by insertion of the same potassium bicarbonate-water charge as employed in Example I.

The above examples are merely illustrative and are not to be understood as limiting the scope of this invention which is limited only as indicated by the appended claims.

What is claimed is:

1. In the method of feeding a liquid oxidizer from a tank containing the same which comprises introducing a hot fuel-rich gaseous mixture into a pressurizing region behind a piston disposed in said tank in contact with said oxidizer said oxidizer comprising a member of the group consisting of red fuming nitric acid, white fuming nitric acid and nitrogen teroxide, said fuel-rich gaseous mixture being produced by combustion of a fuel-rich gas-generator composition whereby said piston displaces said oxidizer, the improvement which comprises placing an inert-gas releasing material in said chamber prior to'introducing said fuel-rich gaseous mixture, said inert-gas-releasing material comprising a material selected from the group consisting of alkali metal carbonates, alkali metal alums, alkali metal oxalates, alkali metal azides and nitrides and mixtures thereof with a minor proportion of water, whereby when said fuel-rich gaseous mixture is introduced into said chamber, said inert gas will be released from said inert gas-releasing material by being heated by said fuelrich gaseous mixture and dilute said fuel-rich gaseous mixture.

2. The improvement of claim 1 wherein said inertgas-releasing material is potassium bicarbonate.

3. The improvement of claim 1 wherein said fuel-rich gaseous mixture contains to weight percent reactive gases in the group consisting of carbon monoxide, hydrogen and methane.

4. The improvement of claim 1 wherein said inert-gasreleasing material is provided in said chamber in an amount suflicient to produce an inert gas concentration of at least 60 mole percent in the mixture produced by contact with said fuel-rich gaseous mixture.

References Cited UNITED STATES PATENTS 2/1959 FOX 6039.48 2/1961 Cumming 60-3948 

